1. Field of the Invention
The present invention relates to an inkjet printing apparatus. In particular, the present invention relates to an inkjet printing apparatus that, with use of an inkjet print head provided with a printing element substrate on which electrothermal transducing elements are arrayed, prints an image while detecting temperature of the printing element substrate.
2. Description of the Related Art
An inkjet print head provided with electrothermal transducing elements can eject small droplets of ink at a high frequency, and a printing apparatus using such a print head can output an image at high sped and high resolution. In the inkjet print head provided with the electrothermal transducing elements, a voltage pulse is applied to each of the electrothermal transducing elements (heaters) according to an ejection signal to make the electrothermal transducing element generate heat. The inkjet print head is configured such that by generating the heat, film boiling occurs in the ink in contact with the electrothermal transducing element, and by growth energy of a generated air bubble, the ink is ejected from an ejection port as a droplet.
In such an inkjet print head, temperature of a printing element substrate on which the plurality of electrothermal transducing elements are arrayed is changed depending on a drive frequency, i.e., an ejection frequency of each of the electrothermal transducing elements. Also, a size of a bubble in the electrothermal transducing element, i.e., an amount of the ink ejected from a corresponding ejection port (ejection amount) depends on the temperature of the printing element substrate. On the other hand, the ejection amount from the electrothermal transducing element is also changed depending on a pulse shape of a voltage pulse applied to the electrothermal transducing element. From the above, many of inkjet print heads each provided with electrothermal transducing elements are adapted to adjust a pulse shape applied to each of the electrothermal transducing elements according to detected temperature of a printing element substrate, and keep a stable ejection amount independently of the temperature of the printing element substrate.
For example, Japanese Patent Laid-Open No. 2000-85128 discloses a method for estimating temperature of the whole of one printing element substrate on the basis of an output result from a plurality of temperature sensors provided on the printing element substrate. An output value from a temperature sensor is largely influenced by a drive frequency of an electrothermal transducing element positioned near the temperature sensor, and therefore it is difficult to obtain average temperature of the whole of a printing element substrate from a detection result of one temperature sensor. As disclosed in Japanese Patent Laid-Open No. 2000-85128, depending on not only detected temperatures of the plurality of temperature sensors but positions of the temperature sensors, a drive frequency within a predetermined period of time, and a driven electrothermal transducing element density, corrections of the detected temperatures of the temperature sensors are made, and thereby more accurate temperature of the printing element substrate can be obtained.
Meanwhile, in a serial type inkjet printing apparatus in which a length of one print scan is long, a rise in temperature of a printing element substrate during the scan is large, and during the one scan, ejection amount control as described above is required. In this case, control including detecting the temperature of the printing element substrate and feeding back the detected temperature to each electrothermal transducing element has to be performed during one scan of a print head.
Also, a printing element substrate used for an inkjet print head is electrically connected to the outside through a flexible wiring board. For this reason, on the flexible wiring board, driving voltage wiring for applying driving voltage to each of an electrothermal transducing elements on the printing element substrate, logic wiring for sending a driving signal, wiring for providing an output from a temperature sensor are mutually adjacently provided.
During a scan of the inkjet print head, each of the electrothermal transducing elements on the printing element substrate is energized by on/off of the driving signal, and current flows through the driving voltage wiring provided on the flexible wiring board. If large current flows through the driving voltage wiring, electromagnetic inductive noise is superimposed on an adjacent temperature sensor output signal line on the flexible wiring board. As a result, the noise is superimposed on an analog signal resulting from temperature detection to prevent accurate temperature detection, and therefore ejection control becomes unstable, which causes an adverse effect such as density unevenness on an image.
In order to solve such a problem, for example, Japanese Patent Laid-Open No. 2002-264305 discloses a method that monitors a temperature sensor provided on a printing element substrate at a time when a driving signal to an electrothermal transducing element is in a disabled (off) state. According to the method disclosed in Japanese Patent Laid-Open No. 2002-264305, when an analog signal serving as temperature information is transmitted, the driving signal is not transmitted, so that no interference occurs between them, and noise is also not added to the analog signal.
However, as in recent years when further increases in speed and resolution are required, even in the case of using the method disclosed in Japanese Patent Laid-Open No. 2002-264305, it is difficult to effectively obtain temperature of a printing element board. For example, in a situation where the increase in speed and improvement in image quality are required, printing is performed at high-resolution ejection timing, and therefore it is difficult to ensure a time when all of driving signals applied to electrothermal transducing elements on a printing element substrate come into the disables (OFF) state. In particular, in the case of performing ejection at high density, a temperature rise of the printing element substrate is drastic, and more frequent monitoring of a temperature sensor is required; however, nevertheless, the time when a driving pulse is disabled (turned OFF) is further limited.
Also, as disclosed in Japanese Patent Laid-Open No. 2000-85128, in the case where a plurality of temperature sensors are provided on one printing element substrate, a wiring length from each of the temperature sensors to a part that is provided on the printing element substrate and connected to a printing apparatus main body is different for each of the temperature sensors. That is, an amount of noise superimposed in a wiring path to the connecting part is different for each of the temperature sensors, and information on a temperature sensor having a longer wiring length becomes less accurate than that of a temperature sensor having a shorter wiring length. Also, an appropriate temperature correction process for removing the noise is required for each of the temperature sensors, and therefore much more processing time is required to obtain temperature.
As described, in the situation where the increases in speed and resolution, it is still difficult to highly accurately obtain temperature of a printing element substrate during a print scan, and on the basis of the obtained temperature, perform appropriate ejection amount control.